Whilst looking for information about trailers I came across the following information.
This information is given as a guide only and needs confirmation in law.

The Law for Trailers

ALL TRAILERS MANUFACTURED AFTER OCTOBER 28, 2012.

New laws that apply to almost every trailer completed after 28-10-12 mean that very few new trailers can be sold or used in the UK unless they have an approval certifcate. This change has been brought about by a vehicle quality standardisation programme instituted by the United Nations, requiring whole vehicle type approval on new trailers made or sold in the UK. These rules have been in place in mainland Europe for many years.

What it means to us in the UK is the end of cowboy trailer building. Every part of the manufacturing process and every bought-in part used to assemble a trailer now has to pass stringent quality controls.

There are two main ways of achieving this approval.

The larger manufacturers are expected to have sought and gained whole vehicle type approval certifcation for their trailers in much the same way as Jaguar, BMW or Ford have been granted type approval certificates for the cars they make.

Smaller and specialist manufacturers, like ourselves, who do not have huge production runs of identical trailers, are more likely to go down the individual vehicle approval route. We, and companies like us, will submit every trailer we make for inspection by the UK Government's Vehicle and Operator Services Agency. VOSA's inspectors will go through every trailer with a fine-tooth comb, checking the quality of construction and of the materials used. They must also check that every bought-in component, such as brakes, axles, lights and hitches, has proper EC approval.

The only trailers that do not need to be submitted for approval are stock trailers that were completed before October 29, 2012 or special purpose trailers. One-off trailers that are designated "special purpose" fall under the testing regime from the end of October 2014.

As we understand the law, the approval rules also apply to trailers built by a person for his or her own use.

Towing in the United Kingdom

Size

Unless you are towing with a vehicle equipped with an air brakes system that is capable of being connected to the trailer, the heaviest trailer allowed on UK roads is 3500kg gross wt.

Maximum length (excluding coupling and drawbar) 7 metres

Maximum width 2.55 metres

Check the manufacturer's recommended towing weight limit for your vehicle. This should be in the handbook and on the VIN plate on the chasis.
Unbraked trailers

No unbraked trailer is allowed to have a plated gross vehicle weight* of more than 750 kg. No unbraked trailer is allowed to have a plated gross vehicle weight that is more than half the kerb weight of the vehicle that is towing it.

*The combined weight of the trailer and the maximum amount of cargo it is designed to carry.

Braked Trailers

Although trailers with overrun brakes can weigh up to 3500kg gross, actual maximum weights are set by the specifications of the vehicle that is doing the towing. The law says you must not exceed the Gross Train Weight that the manufacturer has set for the towing vehicle. Look in the vehilce handbook or on a plate riveted to the vehicle to find this figure. You then have to subtract the weight of the towing vehicle (including fuel, driver, passengers, luggage or cargo) from the Gross Train Weight. The amount you are left with is the maximum theoretically-possible weight of trailer that can be towed legally. REMEMBER, the law does not care whether the trailer is empty or packed to the roof with bricks, what counts is the plated gross vehicle weight of the trailer.

Loading

Your load must be securely tied down.

Check your load doesn’t exceed the trailer's specification.

Loads should be evenly distributed and recommended nose weight limit should not be exceeded.
Load projections should be avoided to minimise risk to others.
Lighting for Trailers

Trailers must have two red sidelights, two red stop lights, a number plate light, two triangular red reflectors and amber indicators (which flash between 60 and 120 times per minute) at the rear. Trailers over 1.3m wide must also have one fog lamp, mounted either in the centre of the vehilce or to the right of centre. Front reflectors are required for trailers less than 1.6m wide and front position lights for trailers wider than 1.6m.

Number Plates

Trailers are required to have legal number plates manufactured by a licensed number plate manufacturer.

Couplings

Trailers should be fitted with 50mm ball coupling to ISO and BSI standards.

Unbraked trailers must have a stout secondary coupling, such as a chain, which is connected securely to the towing vehicle when it is being towed. The secondary coupling must be tight enough to prevent the trailer's tow hitch from hitting the ground if the vehicle becomes uncoupled.

Braked trailers must be fitted with hydraulically damped coupling and auto reverse brakes to give braking efficiencies required by EEC Directive 71/320. All wheels must be braked. Braked trailers must be fitted with a breakaway cable. This must be attached to the towing vehicle in such a manner so that, should the trailer become detached, the breakaway cable will operate the trailer's brakes. It is not advisable to connect the breakaway cable to the towball itself, unless it cannot be avoided. Most tow bars have either a drilled hole, or pigtail attachment, specifically intended to accept the breakaway cable's spring clip.

Braked trailers must be fitted with a parking brake that operates on at least two road wheels on the same axle.

Speed

The maximum speed limit for trailers is 60mph on motorways and dual carriageways. The limit on other roads is 50mph unless a lower road speed limit is in operation. Trailers are not permitted in the outside lane of motorways.

Drivers Licence

Holders of driving licences issued before July 1996 and providing that they have Group A or if after 1990, category B, are entitled to drive a vehicle and trailer combination up to a maximum train weight of 8.25 tons.

However holders of car licences issued after July 1996 will only be able to drive a vehicle and trailer combination of up to 3500kg assuming that the trailer and its load are lighter than the towing vehicle. ie. a 2000kg vehicle with a 1500kg trailer. To tow a larger trailer wighing up to 3500kg a B + E licence must be obtained.
Tachograph

If you are towing for commercial reasons and the combined potential weight of your vehicle and your trailer exceeds 3500kg you should have a tachograph fitted to your vehicle.
Towing vehicle

Having ensured that the towing vehicle is suitable size for the trailer, ensure that the towing bracket is of an approved type and is properly secured. A 50mm diameter towing ball is mormally required to British Standard BS AU113L 1979 or ISO Standard 1103, this will have a flat top which is stamped ISO 50.

If the towing jaw is used it must comply with the requirements of BS AU 24 1964

The vehicle coupling must be at a height which will permit the trailer to stand level when on level ground and the towing vehicle should be fitted with a standard 7 pin electrical socket type 12N wired standard as follows: